A method for improving the strength of a weak basal plane textured magnesium-rare earth alloy

Abstract

The application discloses a method for improving the strength of a weak base surface texture magnesium-rare earth alloy, which comprises the following steps: 1) pre-deforming a magnesium-rare earth alloy plate along the transverse direction with a deformation amount of 4% to 6%; and 2) annealing the pre-compressed plate at a low temperature of 140 DEG C to 160 DEG C for 12 to 24 hours. The magnesium-rare earth alloy plate is treated by pre-deformation and annealing process, so that the strength of the magnesium-rare earth alloy plate is significantly improved, wherein the yield strength and the tensile strength are respectively increased by 53% and 19%, and meanwhile, the plasticity can be basically maintained. The application lays a solid foundation for producing high-strength magnesium-rare earth alloy products, and greatly widens the application prospect of the magnesium-rare earth alloy.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of magnesium alloy sheet processing, in particular to a method for improving the strength of a weak base texture magnesium rare earth alloy. BACKGROUND

[0002] The greenhouse effect will cause global disasters such as climate anomalies, land desertification and food reduction. The overuse of fossil fuels as energy and the emission of a large amount of greenhouse gases such as carbon dioxide are the main causes of the greenhouse effect. Automobile lightweighting can significantly reduce carbon dioxide emissions and is an important measure to prevent the greenhouse effect. As the lightest metal structural material, magnesium alloy has great potential and broad prospects in realizing automobile lightweighting, with a density about one fourth of steel and two thirds of aluminum alloy. However, during the plastic deformation of traditional deformed magnesium alloys such as AZ31 and ZK60, basal slip and tensile twinning are the dominant deformation mechanisms due to their low critical shear stress, but neither of them can coordinate c-axis strain, and although pyramidal slip can coordinate c-axis strain, the high critical shear stress value limits its activation amount. Therefore, the poor room temperature plasticity of traditional deformed magnesium alloys limits their application range and application prospect. The addition of rare earth elements (such as Y and Gd) can effectively reduce the critical shear stress of pyramidal slip in magnesium alloys, thereby promoting the activation of pyramidal slip and improving the room temperature plasticity. As a new type of high plasticity magnesium alloy, magnesium rare earth alloy greatly widens the application range and application prospect of magnesium alloy.

[0003] However, the addition of rare earth elements also causes the weakening of texture, thereby causing the strength of magnesium rare earth alloy to decrease significantly. Professor Zhaoxuan Wu of the Swiss Federal Institute of Technology in Lausanne published a research paper entitled "Mechanistic origin and prediction of enhanced ductility in magnesium alloys" in Science, which found that the tensile yield strength of Mg-1Y and Mg-3Y is even lower than that of pure magnesium. The Jiang Zheng research group of Chongqing University pointed out in "The deformation modes and transferability during low-cycle fatigue of Mg and Mg-3Y alloy" that the main reason for the lower yield strength of Mg-3Y alloy than that of pure magnesium is the weak base texture of Mg-3Y. The lower strength limits the application of magnesium rare earth alloy in high-strength service environments, and the strengthening of weak base texture magnesium rare earth alloy is an important scientific and engineering problem that needs to be solved urgently.

[0004] Currently, the strengthening methods of weak basal texture magnesium-rare earth alloy are mainly increasing the content of rare earth elements, long period stacking ordered phase (LPSO) strengthening, and severe plastic deformation (SPD) induced fine-grain strengthening. A large number of studies have shown that increasing the content of rare earth elements can increase the solid solution strengthening effect of rare earth elements. Dr. Shuaishuai Liu of Chongqing University found in the article "Improving mechanical properties of heterogeneous Mg-Gd alloy laminate via accumulated extrusion bonding" that the yield strength of Mg-Gd alloy increased from 83 MPa to 195 MPa as the content of Gd increased from 1% to 13%. Adding transition metal elements (such as Zn, Ni, etc.) to magnesium-rare earth alloys can form LPSO phases and effectively improve their strength. Koji Hagihara et al. found in the article "Strengthening mechanisms acting in extruded Mg-based long-period stacking ordered (LPSO)-phase alloys" that adding Zn to Mg 98 Y2 (atomic ratio) alloy can obtain Mg 97 Zn1Y2 alloy, which contains 24% volume ratio of 6H LPSO phase in the alloy, the yield strength increased from 150 MPa to 270 MPa. In terms of fine-grain strengthening, Professor Huang Xiaoxu of Chongqing University prepared Mg-5Y alloy fine-grain samples (average grain size of 1.2 μm) by four-pass equal channel angular pressing (ECAP) at 300°C. Compared with the original coarse-grain sample (average grain size of 9.2 μm), the yield strength increased by 24.5 MPa, and the study found that the increase in yield strength was due to the more grain boundaries in the fine-grain sample hindering the movement of dislocations. Professor Huang Xiaoxu of Chongqing University prepared Mg-3Gd fine-grain samples (average grain size of 3.3 μm) by accumulative roll-bonding (ARB) method. Compared with the original coarse-grain sample (average grain size of 45 μm), the yield strength increased from 93 MPa to 173 MPa, and the study found that the high-activity non-basal dislocation slip in the fine-grain sample was the main reason for the increase in yield strength.

[0005] Although the LPSO strengthening, SPD induced fine-grain strengthening can effectively improve the strength of the weak basal texture magnesium rare earth alloy by increasing the content of rare earth elements, there are still some problems: (1) increasing the content of rare earth elements and LPSO strengthening need more rare earth elements and transition metal elements, which are expensive, increase the cost of materials, and the complex metallurgical smelting process increases the preparation cost, and the addition of alloying elements is not conducive to the recycling of magnesium alloy. (2) SPD induced fine-grain strengthening has good strengthening effect, but the SPD processing flow is complex, the processing environment is harsh, and it is not suitable for batch industrial production, and the output size sample is small, which is not suitable for actual service environment. (3) Increasing the content of rare earth elements, LPSO strengthening, SPD induced fine-grain strengthening can improve the strength to a certain extent, but the plasticity of magnesium alloy is reduced, especially LPSO strengthening, which can significantly reduce the plasticity of magnesium alloy. Therefore, it is urgent to explore a strengthening method for weak basal texture magnesium rare earth alloy which can be applied to industrial application. SUMMARY

[0006] In view of the above problems existing in the prior art, the purpose of the present application is to provide a method for improving the strength of weak basal texture magnesium rare earth alloy, which solves the problems of low strength of weak basal texture magnesium rare earth alloy and complex strengthening process flow and high production cost in the prior art.

[0007] In order to solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0008] A method for improving the strength of weak basal texture magnesium rare earth alloy, comprising the following steps:

[0009] (1) compressing the magnesium rare earth alloy plate along the transverse direction to deform by 4% to 6%;

[0010] (2) annealing the pre-compressed plate at a low temperature of 140-160℃ for 12-24 hours.

[0011] The pre-compression deformation is carried out along the transverse direction, which can increase the number of twinning generation, thereby enhancing the strengthening effect. After pre-deformation, low temperature annealing can make the precipitated rare earth atoms segregate at the twin boundaries, hinder part of the dislocations from passing through the twin boundaries, and achieve the effect of pinning strengthening; and the low temperature annealing can ensure that the plasticity of the magnesium rare earth alloy is basically unchanged, and the cost is also lower.

[0012] Further, the magnesium rare earth alloy plate is prepared by the following method: the magnesium rare earth alloy ingot is hot extruded into a plate, the mold temperature is 450℃, the preheating time is 2-3 hours, the extrusion temperature is 450℃, and the extrusion ratio is 18.3-45.7.

[0013] Further, the magnesium-rare earth alloy plate is rough ground by No. 400, No. 600 and No. 800 sandpaper in sequence before compression pre-deformation, and then is fine ground by No. 1000, No. 1200 and No. 2000 sandpaper. In this way, micro cracks can be prevented from being formed at surface defects during pre-compression deformation.

[0014] Further, the compression temperature of transverse compression pre-deformation is 25 DEG C, and the compression strain rate is 1*10 -3 s -1 .

[0015] Further, before low-temperature annealing, the pre-compression plate is wrapped with aluminum foil. In this way, surface oxidation layer can be prevented from being generated during annealing.

[0016] Further, during compression pre-deformation, the magnesium-rare earth alloy plate is clamped by a clamp. In this way, the plate can be prevented from being buckled during compression.

[0017] Further, after low-temperature annealing, the annealed plate is cooled in air. By using air cooling, quenching stress can be reduced, plasticity can be improved, and special quenching medium is not needed, so that cost is saved.

[0018] Further, the magnesium-rare earth alloy plate is a magnesium-yttrium alloy plate, a magnesium-gadolinium alloy plate or a magnesium-cerium alloy plate, and the mass percentage of yttrium, gadolinium or cerium is 1% to 10%.

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] 1. By combining pre-compression deformation and low-temperature annealing process, the yield strength and tensile strength of the plate are significantly improved, the tensile yield strength and tensile strength are increased by 68 MPa and 38 MPa respectively, and are relatively increased by 53% and 19% respectively, so that the problem of low strength of the weak basal texture magnesium-rare earth alloy plate is solved to a great extent. The improvement of the tensile yield strength also makes the processed sample more suitable for tensile stress environment. The present application lays a solid foundation for producing high-strength magnesium-rare earth alloy, and greatly broadens the application prospect of the magnesium-rare earth alloy. Moreover, the process flow of the present application is simple, large-size workpieces can be produced, batch industrial production can be realized, production cost is low, and the present application has high actual industrial application value and excellent economy.

[0021] 2. The deformation process similar to the present application generally reduces plasticity greatly while improving strength, but the pre-compression combined with the subsequent low-temperature annealing process provided by the present application successfully improves the yield strength and tensile strength of the magnesium-rare earth alloy plate while reducing the plasticity by only 3.8%, so that the present application has high engineering practical value.

[0022] 3、The present application can achieve the effect of texture strengthening by introducing the twin crystal with strong wire texture into the matrix with weak base surface texture, and the twin crystal boundary can divide the grain, thereby achieving the effect of fine-grain strengthening. The subsequent low-temperature annealing can ensure that the solute atoms are aggregated on the twin crystal boundary to hinder the movement of dislocations, thereby achieving the effect of pinning strengthening. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 A schematic diagram of the pre-deformation and subsequent annealing process of the weak base surface texture magnesium rare earth alloy plate of the present application;

[0024] Figure 2 The microstructure and {0002} pole figure of different embodiment groups are shown in (a, b) for example 1 and (c, d, e) for example 3. DETAILED DESCRIPTION

[0025] The specific embodiments of the present application will be further described in detail below in combination with specific examples.

[0026] The numerical ranges in the present application should be understood to include each and every intervening value, including the upper and lower limits of the ranges. Intervening values between any stated value or range, as well as any other stated value or intervening value in the stated range, are also included. The upper and lower limits of these smaller ranges can independently be included or excluded in the range.

[0027] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In the case of conflict between the present specification and any document incorporated by reference, the present specification will control. Reference to "comprise", "comprising", "containing", "having" or "include" or variations as used herein is to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0028] The experimental methods used in the present application are conventional methods unless otherwise specified.

[0029] The materials, reagents, etc. used in the present application can be purchased or synthesized by known methods unless otherwise specified.

[0030] In the quantitative test in the present application, three repeated experiments are set, and the average value is taken.

[0031] Reference Figure 1 A method for improving the strength of a weak base surface texture magnesium rare earth alloy, comprising the following steps:

[0032] The magnesium-yttrium alloy ingot was hot-extruded into a plate, and then the extruded plate was pre-compressed in the transverse direction. The pre-compressed plate was then annealed at low temperature, and finally, the plate was tested by unidirectional tensile test at room temperature. The pre-compression deformation was calculated based on the strain at the end of the elastic deformation as the starting point.

[0033] Example 1

[0034] The present embodiment provides a method for preparing a magnesium-rare earth alloy plate, comprising the following steps:

[0035] A magnesium-yttrium alloy ingot with a diameter of 80 mm, in which the mass percentage of yttrium is 3%, was hot-extruded into a plate. The mold temperature was 450°C, the preheating time was 2-3 hours, the extrusion temperature was 450°C, and the extrusion ratio was 18.3.

[0036] Performance test:

[0037] The obtained magnesium-yttrium alloy plate was sampled along the extrusion direction of the extruded plate, and tested by unidirectional tensile test at room temperature. The tensile test showed that the yield strength of the extruded magnesium-yttrium alloy plate without pre-deformation and low-temperature annealing treatment at room temperature was 128 MPa, the tensile strength was 199 MPa, and the elongation was 22.3%. Figure 2 (a, b) are the microstructure and {0002} pole figure of the extruded pure magnesium without pre-deformation, which shows that the grains after extrusion are equiaxed grains with an average grain size of 13.6 μm and weak basal texture, and do not contain twins. The maximum texture intensity is 6.6 mud.

[0038] Example 2

[0039] The present embodiment provides a method for improving the strength of a weak basal texture magnesium-rare earth alloy, comprising the following steps:

[0040] (1) Preparation of magnesium-rare earth alloy plate: the preparation process is the same as that of Example 1.

[0041] (2) The magnesium-yttrium alloy plate sample was pre-compressed in the transverse direction and annealed at low temperature. Before the transverse compression pre-deformation, the magnesium-yttrium alloy plate was first coarsely ground with No. 400, No. 600, and No. 800 sandpaper, and then finely ground with No. 1000, No. 1200, and No. 2000 sandpaper. Then, the plate was pre-compressed in the transverse direction on a CMT5105 universal testing machine. The compression deformation was 4%, the compression temperature was 25°C, and the compression strain rate was 1*10 -3 s -1 In order to ensure that the plate does not buckle during compression, the plate was fixed with a clamp for compression test. After pre-compression deformation, the plate was wrapped with aluminum foil and annealed at 140°C for 24 hours, and then air-cooled after annealing.

[0042] The magnesium-yttrium alloy sample obtained in Example 2 was subjected to uniaxial tensile test at room temperature. The tensile test showed that the strength of the sample was improved, the yield strength was 183 MPa, the tensile strength was 222 MPa, and the tensile elongation was 18.6%.

[0043] Example 3

[0044] This example provides a method for improving the strength of a weak basal plane textured magnesium-rare earth alloy, which has the same preparation process as Example 2, except that the pre-compression deformation is 5%.

[0045] The microstructure and {0002} pole figure of the magnesium-yttrium alloy sample treated by Example 3 are shown in Figure 2 (c, d, e), compared with Example 1, a large number of tensile twins appear in the microstructure, the average grain size is 8.8 μm, 53% of the matrix is the same as the initial material in Example 1, showing weak basal plane texture, the maximum texture intensity in the {0002} pole figure is 6.5 mud, but 47% of the twins show strong fiber texture with the c-axis parallel to the transverse direction, the maximum texture intensity in the {0002} pole figure is 9.8 mud.

[0046] The magnesium-yttrium alloy sample obtained in Example 3 was subjected to uniaxial tensile test at room temperature. The tensile test showed that the strength of the sample was improved, the yield strength was 196 MPa, the tensile strength was 237 MPa, and the tensile elongation was 18.5%.

[0047] Example 4

[0048] This example provides a method for improving the strength of a weak basal plane textured magnesium-rare earth alloy, which has the same preparation process as Example 2, except that the compression deformation is 6%.

[0049] The magnesium-yttrium alloy sample obtained in Example 4 was subjected to uniaxial tensile test at room temperature. The tensile test showed that the strength of the sample was improved, the yield strength was 196 MPa, the tensile strength was 223 MPa, and the tensile elongation was 15.0%.

[0050] Table 1 Comparison table of pre-deformation process, yield strength, tensile strength and elongation of Examples 1-4

[0051]

[0052]

[0053] It can be known by the analysis of examples 1-4 and table 1 that: the weak basal texture magnesium-rare earth alloy can be significantly strengthened by the transverse pre-compression deformation and subsequent annealing treatment process of the application, and the yield strength and tensile strength thereof are obviously improved, and the problem of low strength of the weak basal texture magnesium-rare earth alloy is solved. The main reasons for the strengthening can be attributed to the texture strengthening and fine-grain strengthening caused by tensile twinning and the pinning strengthening caused by the segregation of rare earth atoms at the twin boundary. The tensile twinning can convert the soft-oriented matrix into hard-oriented twin due to the fixed 86° orientation relationship between the matrix and the twin, so as to achieve the effect of texture strengthening. On the other hand, the twin boundary can divide the grains and hinder the sliding of dislocations, so as to achieve the effect of fine-grain strengthening. Moreover, the rare earth atoms will segregate at the twin boundary during low-temperature annealing, hindering part of the dislocations from passing through the twin boundary, so as to achieve the effect of pinning strengthening. At the same time, the processing technology of the application is suitable for manufacturing large-size magnesium alloy workpieces, and the process flow is simple, does not require strict processing environment, is easy for industrial batch production, and has high industrial application value.

[0054] The application only takes the typical magnesium-yttrium alloy (containing 3% yttrium) extruded plate as the implementation object, so as to prove the obvious effect of the application on improving the strength through the comparison of materials and processes; but it does not exclude that the process method of the application is also suitable for the strengthening of other weak basal texture magnesium-rare earth alloys, such as magnesium-gadolinium alloy and magnesium-cerium alloy. Moreover, the above-mentioned is only the preferred implementation mode of the application, and the deformation amount of the pre-compression deformation of the application is not limited to the numerical values of the examples, such as 2%, 3%, and 7%; and the annealing temperature and annealing time of the low-temperature annealing treatment are also not limited to the numerical values of the examples, such as 140℃-12 hours, 150℃-12 hours, 150℃-24 hours, 160℃-12 hours, and 160℃-24 hours. It should be pointed out that, for ordinary skilled persons in the technical field, a number of improvements and refinements can be made without departing from the principles of the application, and these improvements and refinements should also be regarded as the protection scope of the application.

Claims

1. A method for improving the strength of a weak basal texture magnesium rare earth alloy, characterized in that: The steps include: (1) Pre-deform the magnesium rare earth alloy plate by compressing it in the transverse direction, with the deformation amount being 4% to 6%; (2) annealing the pre-compressed sheet at a low temperature of 140°C to 160°C for 12 to 24 hours; The magnesium rare earth alloy plate is a magnesium yttrium alloy, a magnesium gadolinium alloy or a magnesium cerium alloy plate, and the mass proportion of yttrium, gadolinium or cerium is 1% to 10%.

2. The method for improving the strength of a weak basal texture magnesium rare earth alloy according to claim 1, characterized in that: The magnesium rare earth alloy plate is prepared by the following method: the magnesium rare earth alloy ingot is cast and hot extruded into a plate, the mold temperature is 450°C, the preheating time is 2-3 hours, the extrusion temperature is 450°C, and the extrusion ratio is 18.3-45.

7.

3. The method for improving the strength of a weak basal texture magnesium rare earth alloy according to claim 1, characterized in that: The magnesium rare earth alloy plate is coarsely ground with sandpapers No. 400, No. 600 and No. 800 before being compressed and pre-deformed, and then finely ground with sandpapers No. 1000, No. 1200 and No. 2000.

4. The method for improving the strength of a weak basal textured magnesium-rare earth alloy according to claim 1, wherein: The compression temperature of the transverse compression pre-deformation is 25℃, and the compression strain rate is 1*10 -3 s -1 .

5. The method for improving the strength of a weak basal textured magnesium-rare earth alloy according to claim 1, characterized in that: Before low-temperature annealing, the pre-compressed sheets were wrapped with aluminum foil.

6. The method for improving the strength of a weak basal texture magnesium rare earth alloy according to claim 1, characterized in that: During compression pre-deformation, the magnesium-rare earth alloy plate is clamped by a clamp.

7. The method for improving the strength of a weak basal textured magnesium-rare earth alloy according to claim 1, characterized in that: After low temperature annealing, the annealed sheet is cooled in air.

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